Abstract--This paper reports experiment on the previously proposed instantaneous value voltage control of a single-phase ac chopper. The proposed control method is based on a modified carrier SPWM and instantaneous value voltage and current feedback control system. The validation of the proposed method is confirmed by experimental results.

Index Terms—AC chopper, automatic voltage regulation, instantaneous value voltage control, pulse-width modulation.

I. INTRODUCTION Recently, the studies of AC chopper for ac voltage

control has been increased because it has many advantages such as high input power factor, sinusoidal output voltage with small low-pass filter requirement and fast response.

The applications of ac chopper for automatic voltage regulator (AVR) or line conditioning have been proposed based on series voltage compensating technique using auxiliary series transformer connected between supply and load [1]-[3]. The ac chopper-based AVR system can be depicted as shown by Fig. 1. If a single-polarity ac chopper is used the output voltage can only be regulated against one direction supply voltage change i.e. under- or overvoltage while a reversible polarity ac chopper can provide voltage regulation for both supply over- and undervoltage. AC chopper based AVR without series transformer that is capable of compensating for both under- and overvoltage has also been proposed using the buck-boost type ac chopper [4].

Recently, concern about power quality, especially the voltage sags and swells, has been largely increased, due to the prolification of sensitive equipment such as ajustable speed drives, programmable logic controller and process control equipment. The ac chopper based AVRs are considered as alternative solutions to these problems. However, the sudden change in supply voltage magnitude as in cases of voltage sags and swells requires that the AVR should have dynamic response as fast as the inverter-based voltage sag correcting devices currently available. To address this requirement a fast peak voltage detecting technique has been used in the feedback control of the output voltage of the ac choppers [1], [2] and [4]. This technique has limitations of dynamic response and waveform quality under low power factor and non-linear load condition.

The author has proposed a novel instantaneous value voltage control for a single-phase ac chopper in [5]. The proposed control method was based on the modified carrier SPWM and instantaneous value feedback of the output voltage and current signals. The simulation results confirmed that the proposed method provided very fast dynamic response against both sudden supply voltage and load change. The proposed control method also demonstrated nearly sinusoidal output waveform even under nonlinear load.

In this paper, the details and results of the experiment of the proposed instantaneous value control method of the ac chopper are reported.

II. PROPOSED INSTANTANEOUS-VALUE VOLTAGE CONTROL OF AC CHOPPER

A. Configuration of a single-phase ac chopper For a single-phase ac chopper, four switches are used

in the main circuit as shown in Fig. 2 [1]. The output voltage can be controlled by the duty ratio of the chopping pulse. Typical gate signals of the four switches and the chopped output voltage before filtering are shown in Fig. 3. Let the input supply voltage is defined as follows:

cos( )s mv V tω= , (1)

where ω and Vm are the angular frequency and magnitude of the supply voltage, respectively.

The chopped output voltage can be expressed by (2).

Experiment on Instantaneous Value Voltage Control of a Single-Phase AC Chopper

S. Polmai*, and E. Sugprajun* * Faculty of Engineering, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand

Fig. 1. AC chopper-based automatic voltage regulator

1-4244-0844-X/07/$20.00 ©2007 IEEE. 77

1

cos( )sin cos( )

r m

ms

k

v DV tV kD k t

k

ωπ

ω ωπ

∞

=

= +

+ ±∑, (2)

where D and ωs are the duty ratio and angular switching frequency, respectively. The first term in the righ-hand side of (2) is the fundamental component and the second term is the harmonic content distributing around the switching frequency.

For a high switching frequency, the harmonic content can be filtered out using appropriated small-size low-pass filter. In steady-state, the filtered output voltage of the ac chopper can be approximately expressed as follows:

1cos( )o mv DV tω ϕ= − , (3)

where φ1 denotes the fundamental phase delay due to the low-pass filter.

As a result of (3), the amplitude of the output voltage is linearly controlled by the duty ratio of the PWM signal.

In the recent works, the amplitude of the output voltage is regulated by feedback control of the instantaneous peak value of the output voltage using PI controller [1], [2] and [4]. The controller output which is

a dc signal is then compared to the sawtooth carrier pulse to make PWM signals for the four switches. Owning to the delay in the detection of the peak value of the output voltage, the control of the ac chopper output voltage cannot be achieved with fast response. Furthermore, the feedback system is based on dc signals, which do not reflect the wave-shape quality of the output voltage and the variation of the supply voltage. The control system is thus susceptible to non-linearity of the load current and magnitude change of the power supply.

B. Sine-amplitude modulated sawtooth signal generator Fig. 4 illustrates a simplified schematic diagram of the

sawtooth carrier pulse generator. This circuit uses a non-inverting integrator in stead of an inverting integrator as in the previously proposed circuit [5], however the principle of operation is the same.

The absolute-value of the supply voltage signal is used as the input of the integrator circuit. If the supply voltage is defined by (1), it can be shown as in (4) that the output voltage at t1+T instant is equal to the amplitude of the supply voltage at t1+T /2 instant [5].

1 1( ) cos ( / 2)c mv t T V t Tω+ = + , (4)

where, T is the period of the sawtooth signal. From (4), the amplitude of the sawtooth pulse varies with the amplitude of the T/2-second delayed supply voltage. Because the magnitude of the sawtooth pulse is very low around the zero-crossing of the supply voltage, this could lead to fully-on of the switches and makes it unable to control the output voltage around the zero crossing of the supply voltage. To correct this, small dc voltage is added to the integrator input to increase the slope of the triangle pulse around zero-crossing.

Fig.5 illustrates the experimental results of the modified sawthoot signal comparing to the sinusoidal control signal and the resultant PWM signal. As can be seen from the figure, a constant duty ratio PWM waveform is obtained in the same manner as the conventional dc-sawtooth comparison method. The advantage of this method is that the output voltage is independent of the supply voltage as long as the duty ratio is less than a unit

C. Instantaneous-value voltage control The equivalent circuits of AC chopper during active

Fig. 2. Configuration of single-phase ac chopper

-1

0

1

v s (pu)

0

1

S 1

0

1

S 2

0

1

S 3

0

1

S 4

-1

0

1

time

v r (pu)

Fig. 3. Input voltage (vs), chopped voltage (vr) and corresponding gate signal of the four switches.

Fig. 4. Sawtooth pulse generator

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mode of operation, i.e, the switch S1 is “ON” and the switch S3 is “OFF”, and during freewheeling, i.e, the switch S1 is “OFF” and the switch S3 is “ON” are illustrated by Fig. 6 (a) and (b), respectively. For simplicity, the effect of deadtime is ignored. In Fig. 6, the load is modeled as a current source disturbance Io. Using the state-variable averaging method described in [8], the output voltage can be expressed by

2

( ) ( ) ( )( )

1s f f o

of f f f

DV s sL R I sV s

L C s R C s− +

=+ +

(5)

where Rf, Cf and Lf are the resistance, capacitance and inductance of the low-pass filter, respectively.

The block diagram of the AC chopper is illustrated in Fig. 7. The effect of the output current disturebance can be cancelled out by disturbance observer. The closed loop control system of the output voltage is now proposed as shown in Fig. 8. If the parameters of the model match with those of the actual system, the effect of output current disturbance will be rejected and the closed loop transfer function can be found as

( )* 2

( ) 1( ) 1

o D P

o f f D f f P

V s K s KV s L C s K R C s K

+ +=

+ + + +, (6)

where KD and KP are the derivative and proportional gain of the PD controller, respectively.

The poles of the closed-loop control system are

21,2 1n ns jζω ω ζ= − ± − , (7)

where

1Pn

f f

KL C

ω +=

, (8)

and

( )( )

1 12 1

D f f

P f f

K R CK L C

ζ = ++

(9)

are the natural frequency and damping factor of the closed-loop system, respectively.

The desired closed-loop control characteristics can be specified by properly selecting the gains KD and KP.

III. EXPERIMENTAL RESULTS Fig. 7 illustrates the configuration of the ac chopper

for experiment. The important parameters of the system are summarized in the Table I. The experiments have been carried out to evaluate the performance of the proposed method. Additionally, the experiment results using conventional average-value control method will be used for comparison.

A. Step response The experimental result with the proposed

instantaneous-value control system is illustrated in Fig. 10. The output voltage follows the reference control signal precisely and has very fast dynamic response.

Fig. 5. Measured sawthoot pulse and control signal (upper plot) and resultant PWM signal (lower plot)

(a) Conducing duration DT

(b) Conducting duration (1-D)T

Fig. 6. Equivalent of AC chopper during active mode of operation (a) and freewheeling mode of operation (b).

Fig. 7. Block diagram of AC chopper model

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B. Effect of non-linear load The non-linear load is made of a full-wave rectifier

with LC filter and resistive load. To illustrate the effect of non-linear load, the output voltage without the proposed control method is shown in Fig. 11. The steady-state experimental result with the proposed instantaneous-value control system is illustrated in Fig. 12. Nearly sinusoidal output voltage is obtained.

C. Effect of load change The experimental result on step load change is shown in Fig. 13. The output voltage is not affected by the change of the load.

D. Effect of supply voltage change Fig. 13 illustrates the experimental results when the ac supply has voltage dip to 70% of its nominal voltage for 5 cycles. Fig. 14 illustrates the experimental results when the ac supply has voltage swell to 110% of its nominal voltage for 5 cycles. With the proposed control method the output voltage is nearly unaffected with the suddenly supply voltage change. These results demonstrate the robustness of the proposed modulation method.

IV. CONCLUSIONS In this paper, the validity of the proposed instantaneous

value voltage control for a single-phase ac chopper has been confirmed experimentally. The proposed control method provides fast step response, nearly sinusoidal voltage waveform

TABLE I. IMPORTANT PARAMETERS FOR EXPERIMENTS.

AC chopper Supply voltage 220 V, 50 Hz Switching frequency 20 kHz

Load-side low-pass filter Lf 500 μH Rf 0.05 Ω Cf 5 μF

Supply-side filter Cs 16 μF

Controller parameters KD 0.00005 KP 3.6

Nonlinear load (Diode rectifier with dc reactor)

Filter capacitor 2600 μF Load resistance 18 Ω

DC reactor 15 mH IGBT

S1-S4 G20N60B3D, 600V 40A

Fig. 8. Block diagram for the control system of the output voltage.

Fig. 9 Configuration of the ac chopper system for experiment

Fig. 10 Measured output voltage (upper trace, 100V/div) and reference controlled voltage (lower trace, 5V/div) for step response experiment. (Time scale: 5ms/div)

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under non-linear load condition and robustness against the supply voltage dips.

The proposed modulation and control system can enhance the function and performance of ac chopper-based AVR system to be able to compete with high performance inverter-based voltage sag mitigating devices. Though the experiment was carried out only with a buck-type ac chopper, this control scheme can also be used for other type of ac chopper as well.

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[5] S. Polmai, “Instantaneous-value voltage control of a single-phase AC chopper”, Proc. International Power Electronics Conference, IPEC-Niigata 2005, vol. 49, No. 4, (CD-ROM) April 4 – 8, 2005

Fig. 11.Measured output voltage (upper trace, 100V/div) and load current (lower trace, 1A/div) under nonlinear load condition without the proposed control method. (Time scale: 5ms/div)

Fig. 12.Measured output voltage (upper trace, 100V/div) and load current (lower trace, 1A/div) under nonlinear load condition with the proposed control method. (Time scale: 5ms/div)

Fig. 13. Measured output voltage (upper trace, 100V/div) and load current (lower trace, 1A/div) with step load change. (Time scale: 10ms/div)

Fig. 14. Measured supply voltage (upper trace, 100V/div) and output voltage (lower trace, 100V/div) under supply voltage dip condition. (Time scale: 50ms/div)

Fig. 15. Measured supply voltage (upper trace, 100V/div) and output voltage (lower trace, 100V/div) under supply voltage swell condition. (Time scale: 50ms/div)

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[6] M.J. Ryan; W.E. Brumsickle, and R.D. Lorenz. “Control topology options for single-phase UPS inverters”, IEEE Trans. Industry Applications, Vol. 33, No. 2, March-April (1997)

[7] Josep M. Guerrero, Luis Garcia de Vicuna, Jaume Miret, and Jose Matas. “A nonlinear feed-forward control technique for single-phase UPS inverters”, Proc. the IEEE Conference IECON 02, Vol. 1, pp. 257-261 (2002)

[8] Andre S. Kislovski, Richard Redl, Nathan O. Sokal, “Dynamic Analysis of Switching-Mode DC/DC Converters”, Van Nostarnd Reinhold, NY, 1991

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